1. Field of the Invention
The invention relates to a common rail injector for injecting fuel in a common rail injection system of an internal combustion engine, which system has an injector housing with a fuel inlet that is in communication with a central high-pressure fuel reservoir outside the injector housing and with a pressure chamber inside the injector housing, from which fuel subjected to high pressure is injected as a function of the position of a control valve that assures that a nozzle needle, which is movable back and forth and received in a longitudinal bore of the injector axially counter to the prestressing force of a nozzle spring that is received in a nozzle spring chamber, lifts from a seat when the pressure in the pressure chamber is greater than the pressure in a control chamber that communicates with the fuel inlet via an inlet throttle.
2. Description of the Prior Art
In common rail injection systems, a high-pressure pump pumps the fuel into the central high-pressure fuel reservoir, which is called a common rail. From the high-pressure fuel reservoir, high-pressure lines lead to the individual injectors, which are assigned to the engine cylinders. The injectors are triggered individually by the engine electronics. The rail pressure prevails in the pressure chamber and at the control valve. When the control valve opens, the nozzle needle lifts from its seat counter to the prestressing force of the nozzle spring, and fuel subjected to high pressure is injected into the combustion chamber.
In conventional injectors of the kind known for instance from German Patent Disclosures DE 197 24 637 A1, relatively long nozzle needles are used. In operation, because of the high pressures and the rapid load changes, very strong forces act on the nozzle needle. These forces cause the nozzle needle to be stretched and compressed in the longitudinal direction. This in turn means that the nozzle needle stroke varies as a function of the forces acting on the nozzle needle.
It is the object of the invention to furnish a common rail injector that while using conventional injection nozzles makes markedly higher nozzle needle speeds possible. Furthermore, the injector of the invention should be simple in construction and should be able to be produced economically.
In a common rail injector for injecting fuel in a common rail injection system of an internal combustion engine, which system has an injector housing with a fuel inlet that is in communication with a central high-pressure fuel reservoir outside the injector housing and with a pressure chamber inside the injector housing, from which fuel subjected to high pressure is injected as a function of the position of a control valve that assures that a nozzle needle movable back and forth and received in a longitudinal bore of the injector axially counter to the prestressing force of a nozzle spring that is received in a nozzle spring chamber, lifts from a seat when the pressure in the pressure chamber is greater than the pressure in a control chamber that communicates with the fuel inlet via an inlet throttle, this object is attained in that the control chamber is formed by a cylindrical chamber, in which a control peg, embodied on the end of the nozzle needle remote from the combustion chamber, is displaceable causing a sealing effect, and that the nozzle spring chamber is disposed outside the control chamber, in the region of the end of the nozzle needle remote from the combustion chamber. It is also conceivable to accommodate the nozzle spring in the control chamber. Then, however, the installation space of the nozzle spring can unfavorable impact the function of the injector. To achieve acceptable closing speeds, a high spring rigidity is necessary, which in turn requires a large installation space. This increases the control chamber volume, and the performance of the injector becomes worse. This impairs in particular the definition of the instant of injection and the length of injection. Furthermore, the requisite control quantity is increased, so that the overall efficiency drops and there is a risk of excessive fuel heating. The invention offers the advantage that the control chamber and the nozzle spring chamber can be combined on the end remote from the combustion chamber of the nozzle needle, without the volume of the control chamber depending on the structural space of the nozzle spring. It is therefore possible to build in a nozzle spring with high spring rigidity, which assures good closure of the nozzle needle. As a result, the injection time and the instant of injection can be defined exactly. With the injector of the invention, nozzle needle speeds that are greater than 1 m/s can be attained during opening and closing. Because of the control peg that is displaceable in the control chamber, the nozzle needle diameter can be selected arbitrarily. By separating the functional measure, or length, which for instance can amount to 3 mm, and the production measure, or length, which can for instance amount to 4 mm, production costs are reduced.
A particular type of embodiment of the invention is characterized in that the fuel inlet discharges into the nozzle spring chamber, and that at least one flat face is embodied on the nozzle needle between the nozzle spring chamber and the pressure chamber. As a result of the flat face, a flow connection is created between the nozzle spring chamber and the pressure chamber, through which the fuel to be injected reaches the pressure chamber from the fuel inlet. The bore to the pressure chamber which is present in conventional injectors can be omitted.
A particular type of embodiment of the invention is characterized in that the control chamber is embodied in a valve piece, which has a central outlet bore with an outflow throttle and a valve seat. Through the central outlet bore, a communication between the control chamber and a relief chamber is created. The valve seat cooperates with a control valve member of a 2/2-way valve, which controls the course of injection of the injector of the invention.
Further particular types of embodiment of the invention are characterized in that the inlet throttle is integrated with either the valve piece or the nozzle needle. The inlet throttle can take the form of a bore or a groove. For production and/or cost reasons, one or the other type of embodiment will be preferable.
A particular type of embodiment of the invention is characterized in that the nozzle needle is guided by the control peg. Because of the functional principle according to the invention, no internal leakage at the guide occurs in the unactuated state. This means lower specific consumption values.
A particular type of embodiment of the invention is characterized in that the nozzle needle is guided on its end toward the combustion chamber. The additional guidance of the nozzle needle increases the operating reliability and the service life of the injector.
A particular type of embodiment of the invention is characterized in that a step which forms a stop for a spring plate is embodied on the nozzle needle. The spring plate forms an abutment for the nozzle spring and at the same time forms the stroke stop for the nozzle needle. By the choice of a suitable thickness of the spring plate, both the prestressing force of the nozzle spring and the nozzle needle stroke can be adjusted.